The present invention relates to a display panel, and in particular, to a display panel employed in an organic light emitting display device.
FIG. 1 is a schematic diagram of a conventional organic light emitting display panel. As shown in FIG. 1, a panel 1 comprises a data driver 11, a scan driver 12, and a display array 13. The data driver 11 controls a plurality of data lines DL1 to DLn, and the scan driver 12 controls a plurality of scan lines SL1 to SLm. Interlaced data lines DL1 to DLn and scan lines SL1 to SLm form a display array 13. Each pair of the interlaced data line and scan line corresponds to a display unit. For example, the interlaced data line DL1 and scan line SL1 correspond to a display unit 100. As with any other display unit, the equivalent circuit of the display unit 100 comprises a switch transistor T11, a storage capacitor Cs1, a driving transistor T12, and an organic light-emitting diode (OLED) D1. The driving transistor T12 is a PMOS transistor, for example.
The scan driver 12 sequentially outputs scan signals to the scan lines SL1 to SLm to turn on the switch transistors within all display units corresponding to one row and turn off the switch transistors within all display units corresponding to all other rows. The data driver 11 outputs video signals with gray scale values to the display units corresponding to one row through the data lines DL1 to DLn according to prepared but not yet displayed image data. For example, when the scan driver 12 outputs a scan signal to the scan line SL1, the switch transistor T11 within the display unit 100 is turned on. The data driver 11 then outputs a corresponding video signal to the display unit 100 through the data line DL1, and the storage capacitor Cs1 stores the voltage of the video signal. The driving transistor T12 provides a driving current Id1 to drive the OLED D1 to emit light according to the stored voltage in the storage capacitor Cs1.
Because the OLED D1 is a current-driving element, the brightness of the OLED D1 is determined by the intensity of the driving current Id1. The driving current Id1 is a drain current of the driving transistor T12 and refers to the driving capability thereof. The driving current Id1 is represented by the following equation:id1=k(vsg+vth)2 
where id1, k, vsg and vth represent a value of the driving current Id1, a conductive parameter of the driving transistor T12, a value of the source-gate voltage Vsg of the driving transistor T12, and a threshold voltage of the driving transistor T12 respectively.
Because the driving transistors in different regions of the display array 13 are not electrically identical due to the fabrication process thereof, the threshold voltages of the driving transistors are unequal. When the display units within different regions receive the same video signal, the driving current respectively provided by the driving transistors of the display units is not equal due to the unequal threshold voltages of the driving transistors. Thus, brightness of the OLEDs is not equal, resulting in unequal OLED light-emission intensity in a frame cycle and uneven images displayed on the panel 1.
Referring to FIG. 2, because the driving transistor T12 is a PMOS transistor, an input port 21 of a power line on the panel 1 is coupled to a voltage source Vdd. A person having ordinary skill in the art will recognize that the input port 21 of the power line is coupled to a voltage source Vss when the driving transistor T12 is an NMOS transistor. According to the disposition of the power lines on the panel 1, the display unit, which farther from the input port 21, corresponds to greater equivalent resistance of the power line. Thus, because the display unit is closer to the input port 21, brightness is greater, while the brightness of the display unit farther from the input port 21 is less bright, resulting in unequal brightness.